The present invention relates to apparatuses for radiological examination, and particularly to the portion of such apparatuses used to hold photographic film or plates onto which the image of a part of the body of the patient that is undergoing examination is impressed.
Radiology is commonly known as the medical science that is concerned with the use of ionizing radiation, particularly x-rays, for the purpose of diagnosis and treatment of disease. A part of the body of the patient is substantially exposed to a beam of radiation that moves through the body in a selective manner, and finally impresses an image on the photographic film. An image is thereby provided from which valuable clinical information can be derived concerning the part of the body undergoing examination.
A major problem that is encountered in connection with radiological examination is the scattering of radiation in the body of the patient, which scattering gives rise to interference on the image. That is, the image is intended to be generated by direct radiation, but indirect radiation due to scattering can interfere with this image. As a result, radiological apparatus require the provision of an arrangement that enables the apparatus to minimize the effect of scattered radiation. Such an arrangement is generally known in the art as a Potter-Bucky grid, or a Bucky diaphragm. In practical terms, this is a grid that is formed by an assembly of lead strips, resembling an open Venetian blind, which is placed between the body of the patient that is being x-rayed and the photographic film or plate that is to receive the image. The grid has the task of filtering out the scattered radiation that would otherwise impair the quality of the impressed image.
The rectilinear lead strips of the grid are usually spaced from each other by the interposition of strip of an x-ray transparent material. In the past this was usually wood, but is now commonly replaced by plastics or aluminum. The lead strips are oriented so that they are directed towards a virtual line, that is, the focal line. The focus of the x-ray emitting tube is placed along this focal line. The primary rays, after having passed through the body of the patient, come up against the grid. The strips of the grid oppose the primary rays at just a minimum of the overall absorption surface. Conversely, oblique rays due to scattering by the body of the patient are prevented from passing through the grid by the strips. Thus, the grid carries out selective absorption of the scattered radiation.
Moving grids were first provided by assembling the strips in accordance with the cylindrical surface whose axis coincided with the focal line. During the movement of the grid, the grid remained focused. However, this particular type of embodiment had a major drawback in that examination tables were provided with a cylindrical surface having the grid placed thereunder, or if the surface was plane, in a preferable embodiment, the need arises to place the grid above the surface, having appropriate devices on all sides thereof for the convenience of the patient.
Currently, plane grids are used, which represents major progress. However, these plane grids still have drawbacks in that the focusing of the radiation is not maintained over all of the grid during movement.
The ratio of the height h of a lead strip to the distance d of the lead strip can be said to be R, and equal to h/d. A grid having a high ratio R requires the central ray to be focused in an extremely accurate manner. A centering or focusing error would in fact entail a considerable absorption effect. For example, a two degree focusing error on the grid having a ratio R of 6.5 would entail a 64% transmission for the primary radiation, whereas such a transmission would be further reduced to 37% with a ratio R of 16. In practice, there is a limit to the ratio R for the grids, the limit being set by the need for an acceptable compromise between the anti-scattering effect desired and the highest possible contrast which can be reached.
Another problem that has been encountered in using grids relates to the cancellation of the radiographic image of the grid itself. In order to cancel this image, the grid has been installed in a driving mechanism which confers a reciprocating motion to the grid. It is has been experimentally found that there are reciprocating speeds of the grid that bring about stroboscopic effects for displacements of a step or half a step. An appropriate selection of the speed, in accordance with the exposure time, enables such effects to be avoided, or at least to be minimized to negligible levels. It can be conclusively stated, however, that an overall solution of all of the afore mentioned problems and drawbacks relating to the use of grids has up to now proven to be practically impossible.